Multistable device



3, 1967 R. G. WILLIAMSON 3,334, f

MULTISTABLE DEVICE Filed June 17, 1964 2 Sheets-Sheet 1 F W. 1 ea\ FLUID SOURCE PRESSURE CONTROL F HG. 2b

' F IG 2Q SZAPRESSURE INVENTOR ROBERT G. WILLIAMSON 3, 1957 R. G. WlLLlAMSON 3,334,644

MULTISTABLE DEVICE United States Patent 3,334,644 MULTISTABLE DEVICE Robert G. Williamson, Nor-walk, C0nn., assignor to perry Rand Cor oration, New York, N.Y., a corporation of Delaware Filed June 17, 1964, Ser. No. 375,908 13 Claims. (Cl. 137111) The present invention relates to fluid actuated gating elements having a plurality of stable states. More particularly, the present invention relates to fluid actuated logic devices of the type employing movable pistons and suitable for use in computers and digital control circuits.

In the prior art there is disclosed a logical gating element wherein a relatively thin piston plate is selectively shifted between two stable states, The plate is enclosed in a piston chamber and is selectively shifted from one end of the chamber to the other in response to fluid pressure signals. In this device the piston is limited to a single degree of freedom. That is, the piston may move along only one path as it shifts back and forth between the two stable states.

An object of this invention is to provide a gating element comprising a piston disposed within a chamber, the relative dimensions of the piston and chamber being such that the piston has more than one degree of freedom and can move along more than one path. In a first embodiment two dimensions of the chamber are greater than the corresponding dimensions of the piston while the third dimensions are substantially equal. In this embodiment the piston has two degrees of freedom and moves about in a single plane in response to fluid pressure signals. In a second embodiment all dimensions of the chamber are greater than the corresponding dimensions of the piston whereby the piston has three degrees of freedom and may move about in more than one plane.

An object of this invention is to provide a. gating element comprising a piston disposed within a piston chamber and operative in response to selectively applied fluid pressure signals for assuming any one of three or more stable states.

Still another object of this invention is to provide a logical gating element including a piston which is shifted between two or more stable states in response to fluid signals derived from three or more signal sources.

Yet another object of the invention is to provide a piston element having N working surfaces disposed in N planes, at least two of the planes being non-parallel and N being a whole number greater than two, means defining a piston chamber having N walls each disposed substantially parallel to but spaced apart from a corresponding working surface of the piston when the piston is centered in said chamber, said piston having M stable states in each of which one of said working surface is in cooperative engagement with its corresponding chamber wall, M being a whole number greater than 1 but equal to or less than N, a plurality of means for selectively applying forces against at least some of said working surfaces to move the piston between the stable states, and means for sensing the state of the piston.

A feature of the invention is the provision of means for sensing the position of a multistable piston having more than one degree of freedom. The sensing means may take more than one form. In a first embodiment the sensing means comprises a fluid passage extending through the piston. At one surface of the piston the passage continuously mates with an inlet regardless of the position of the piston. At the opposite surface the passage selectively mates with one of a plurality of outlets depending upon the position of the piston. The position of the piston is sensed by applying a fluid pressure signal to the inlet and sensing the outlets to determine which outlet receives the signal.

The sensing means described in the preceding paragraph is suitable for use only with pistons having one or two degrees, but not three, of freedom. Therefore, a further object of the invention is to provide means for sensing the position of a piston element having either two or three degrees of freedom. In this embodiment the walls of the piston chamber have elongated fluid seals thereon so that the seals, the chamber wall, and the piston surface form a chamber when the piston is driven into cooperative engagement with the wall. At least one inlet and one outlet are connected with the chamber thus formed. The position of the piston is sensed by applying a fluid pressure signal to the inlet and sensing the pressure at the outlet.

Other objects of the invention and its mode of operation will become apparent upon consideration of the fol lowing description taken in conjunction with the accompanying drawings in which:

FIGURE 1 illustrates a multistable logical gating element having two degrees of freedom in combination with means for sensing the state of the gating element;

FIGURES 2A, 2B, 2C, annd 2D are similar views with top plate removed;

FIGURE 3 is a sectional view taken along the line 33 of FIGURE 2A;

FIGURE 4 is a sectional view similar to FIGURE 3 but showing an alternative arrangement of the sensing passage;

FIGURE 5 illustrates a logical gating element having four stable states and two degrees of freedom;

FIGURE 6 illustrates a logical gating element having three degrees of freedom;

FIGURE 7 is a perspective view of a further embodiment of the invention;

FIGURE 8 is a sectional view taken along the line 8-8 of FIGURE 7; and

FIGURE 9 is a sectional view taken along the line 99 of FIGURE 8.

FIGURES 1 through 3 illustrate the basic principles of the invention as applied to a logical gating element having three stable states manifested by three positions of a piston having two degrees of freedom. The gating elements comprises three flat plates 10, 12, and 14 stacked together to form a substantially solid body 16. Within the body 16 is a generally triangularly shaped chamber 18. This chamber is formed by cutting a section of triangular shape out of plate 10 and then inserting the plate 10 between cover plates 12 and 14. The plates are held together in a fluid-tight relationship by any suitable means such as an adhesive or mechanical fasteners.

Disposed within chamber 18 is a triangular shaped piston element 20. For purposes of explanation the piston element is shown in FIGURE 2D as being positioned in the center of chamber 18. The piston has three working surfaces 22, 24, and 26. These surfaces are called Working surfaces because as subsequently explained, fluid pressure signals applied to the chamber work or act against these surfaces to shift the piston from one of its stable states to another. When the piston is centrally positioned within the chamber each of its working surfaces is parallel to but spaced apart from the corresponding wall of the chamber. Thus, working surface 22 is parallel to wall 28, surface 24 is parallel to wall 30', and surface 26 is parallel to wall 32. Each of the Working surfaces 22, 24, and 26 is of substantially the same length as the corresponding chamber wall 28, 30, and 32. Obviously, for any measurement made on a line extending through the center of the chamber and piston, the dimension of the chamber is greater than the dimension of the piston so the piston is free to move about within the plane of plate 10.

As shown in FIGURE 3, the piston has two sliding surfaces 34 and 36 in sliding engagement with surfaces 38 and 40 of the piston chamber. The thickness of the piston is chosen such that there is a minimum clearance between surfaces 34 and 38 and 36 and 40 while still permitting the piston to slide freely within the plane of chamber 18. It should be noted that the thickness of plate and piston have been exaggerated in the drawings for the sake of clarity. In actual practice the plate is made quite thin in order to reduce its mass.

As shown in FIGURE 2A, portions of plate 10 are cut out to form three fluid passages 42, 44, and 46. Passage 42 terminates at an opening in wall 32 and is connected to a pressure control means 48. Passage 44 terminates at an opening in wall and is connected to a pressure control means 50. Passage 46 terminates at an opening in wall 28 and is connected to a pressure control means 52. Pressure control means 48, 50, and 52 may be any suitable means for selectively applying to the corresponding passage a fluid signal of a first or a second level. Each pressure control means may, for example, comprise means for alternately connecting the corresponding channel to a vent or to a vacuum source.

Assume that the piston is in the position shown in FIGURE 2D. If passages 42 and 44 are both vented to the atmosphere and passage 46 is connected to a vacuum source then the resulting low pressure at the bottom of the piston will draw the piston downwardly until its surface 22 moves into engagement with wall 28. This position of the piston is shown in FIGURE 2A and represents one of the three stable states of the piston. This is defined as a stable state because the piston remains in this state for at least as long as passages 42 and 44 are vented and a vacuum is applied to passage 46. Should the plate 10 be disposed in a horizontal plane then the piston will retain this state even after the vacuum is removed from the passage 46 provided passages 42 and 44 are still vented. FIGURES 2B and 20 show the second and third stable states of the piston. In order to move the piston into the position shown in FIGURE 2B pressure control means 50 and 52 connect passages 44 and 46 to the atmosphere while pressure control means 48 applies a suction signal to passage 42. The resulting low pressure in the region between working surface 26 and wall 32 draws the working surface into engagement with the wall.

In order to move the piston into the position shown in FIGURE 2C pressure control means 48 and 52 connect channels 42 and 46 with the atmosphere while pressure control means 50 connects passage 44 to vacuum. The resulting low pressure between working surface 24 and wall 30 draws the working surface into engagement with the wall. 7

In order to sense the position of the piston an inlet passage 69 (FIGURE 3) extends through plate 12 and three outlet passages 51, 53, and 54 (FIGURE 2A) extend through plate 14. A recess 56 is formed in one sliding surface of the piston and a passage 58 connects with the recess and extends through the piston to the other sliding surface. The arrangement is such that recess 56 always connects inlet 69 with passage 58 regardless of the position of the piston. However, the size and location of passage 58 are such that the passage connects With an individual one of the outlets depending on the stable position of the piston. That is, passage 58 communicates with outlet 54 when the piston is in the position shown in FIGURE 2A, connects with outlet 51 when the piston is in the position shown in FIGURE 2B, and connects with outlet 53 when the piston is in the position shown in FIGURE 2C.

Outlets 51, 53, and 54 are connected by means of pipes 60, 62, 64 (FIGURE 1) to any desired output device 67. Inlet 69 is connected by means of a pipe 66 to a source 68 which provides a fluid signal at a desired pressure. Thus, the pressure provided by source 68 is communicated by means of pipe 66, inlet 48, recess 56, and passage 58 to one of the outlets 51, 53, or 54 depending upon whether the piston is in the position shown in FIGURE 2A, FIGURE 2B, or FIGURE 2C.

FIGURE 4 shows a slightly different arrangement of the sensing means. In this arrangement the inlet 69 extends from the outer surface of plate 12 to a recess 156 formed in the inner surface of plate 12. A passage 158 of substantially uniform diameter extends through piston 20 from one sliding surface to the other. Plate 14 has three outlets 51, 53, and 54 extending through it as in the previous embodiment. In this arrangement the recess 156 is made large enough so that it communicates with passage 158 regardless of the position of piston 20. Depending upon the position of piston 20 passage 158 communicates with one of the outlets 51, 53, or 54.

FIGURE 5 shows a second embodiment of the invention employing a piston having four stable states and two degrees of freedom. The general features of construction of this embodiment may be the same as the embodiment previously described. However, the center plate has its center portion removed to form a generally rectangularly shaped piston chamber 118 and has ad ditional portions removed to form four passages 160, 162, 164-, and 166. Each of these passages is connected to a pressure control means which selectively applies suction to the passage or connects it to the surrounding environment. In this embodiment the piston 126 has four Working surfaces 170, 172, 174, and 176 against which fluid forces may be exerted to move the piston to any one of four stable states.

FIGURE 5 shows one stable state of the piston wherein working surface 172 is in engagement with wall 173 of the chamber. The piston may be moved to this position from any of its other stable states by connecting passages 162, 164, and 166 to the atmosphere while applying a suction signal to passage 160. By applying a suction signal to passage 162 and connecting passages 160, 164, and 166 to the atmosphere the piston may be moved to a second stable state with its working surface 174 in engagement with wall 180. By applying a suction signal to passage 164 and venting passages 160, 162, and 166 to the atmosphere the piston may be moved to a third stable state with its working surface 176 in engagement with wall 182. The piston may be moved to its fourth stable state by applying the suction signal to passage 166 while venting passages 160, 162, and 164 to the atmosphere. In the fourth stable state working surface 170 is in engagement with wall 184.

This embodiment employs the sensing system shown in FIGURE 4. Therefore, piston has a passage 158 located at its center and extending through the piston from one sliding surface to the other. The back plate has four outlet passages 186, 188, 190, and 192 extending therthrough and passage 158 is selectively aligned with one of the outlets depending upon Whether the piston is in its first, second, third, or fourth stable state, respectively.

The invention is not limited to gating elements having pistons with three or four stable states but comprehends gating elements employing pistons having up to M stable states where M is a whole number and corresponds to the number of working surfaces on the piston. FIGURE 6 illustrates a logical gating element employing a piston having six stable states and two degrees of freedom. Since the piston has six stable states it is provided with six working surfaces each. of which is parallel to a wall of the piston chamber. The plate 210 has six passages formed therein each connecting with the chamber through one of the walls. Each passage is connected to a pressure control means which selectively vents the passage to the atmosphere or else connects it to a vacuum source. By selectively applying a vacuum signal to one of the passages and venting the remaining passages the piston may be moved to any one of six desired positions. At the center of the piston is a passage 158 which extends through the piston from one sliding surface to the other. Since the piston has six stable states the back plate is provided with six outlet channels and the passage 158 is selectively aligned with one of these outlets depending upon the state of the piston. In view of the similarities between this and the preceding embodiments, a detailed description of FIG- URE 6 is believed unnecessary.

It is anticipated that in most applications the pistons described above will be symmetrical and adjacent working surfaces will intersect at an angle of degrees where N is the number of working surfaces on the piston. However, it is also possible to employ in the present invention pistons which are not symmetrical.

In describing the embodiments shown in FIGURES 2, 5, and 6 it has been assumed that the piston may assume any one of M N stable positions where N is equal to the number of working surfaces. If a gating element is provided with pressure control means for constantly venting K passages then the number of stable states of the piston is reduced to M=NK. This applies to any of the embodiments shown in FIGURES 2, 5 and 6 as well as the embodiments subsequently described with reference to FIGURE 8.

As an illustration, assume that passages 160 and 162 (FIGURE 5) are constantly vented. In this case M=4-2=2 so that the piston 120 has two stable states. The first stable state is obtained by venting passage 164 and applying suction to passage 166 to draw piston surface 170 into engagement with wall 184. The second stable state is obtained by venting passage 166 and applying suction to passage 164 to draw piston surface 176 into engagement with wall 182.

In the previous discussion it has been assumed that the pressure control means such as 48, 50, and 52 (FIGURE 2A) selectively connect passages 42, 44, and 46 to a vacuum source or else vent the passages to the atmosphere.

However, the present invention is not limited to use with this combination of pressures. For example, one of the passages such as passage 46 may be vented and pressure signals above atmospheric pressure applied to the remaining passages 42 and 44. The pressure signals act against working surfaces 24 and 26 to move the piston toward wall 28. Also, the resulting flow of fluid around the piston as indicated by arrows 70 and 72 (FIGURE 2D) reduces the pressure between wall 28 and working surface 22. As a result, the piston moves downwardly until its working surface 22 is in cooperative engagement with wall 28.

From the preceding paragraph it becomes obvious that many combinations of pressure levels, including atmospheric pressure may be used to shift the pistons in the various gating elements described above. Generally speaking, the only requirement is that a pressure signal of a first level be applied to one passage at the same time pressure signals of a second level greater than the first level are applied to the remaining passages. It is of course necessary that the resulting force acting on the piston be sufiicient to move the piston to the desired stable state.

In the preceding embodiments the piston has been limited to movement in a single plane and thus has had only two degrees of freedom. FIGURES 7 through 9 shows a multistable device 116 employing a piston having six stable states and three degrees of freedom. In the specific embodiment shown the device comprises a substantially solid body 310 having formed therein a cubeshaped member 318. Disposed within the chamber is a cube-shaped piston 320. The dimensions of the piston and chamber are such that if the piston were supported in the center of the chamber each of its surfaces would be parallel to but spaced apart from a corresponding wall of the chamber. Therefore, the piston is free to move about in any direction within the volume defined by the chamber walls.

A plurality of inlet and outlet passages extend through each wall and connect with chamber 318. Suitable pipes or other fluid conveying means are connected to these passages for selectively supplying fluid pressure signals to position the piston, and for supplying to and conveying from the chamber pressure signals indicating the position of the piston. FIGURE 8 shows an examplar arrangement of inlets and outlets for four of the walls. As will be obvious from the subsequent description the inlet and outlet arrangements for the other two walls may be similar to those shown.

In FIGURE 8, passages 322, 324, 326, and 328 are control signal passages. In addition, the two walls not shown are each provided with a control signal passage. These passages are called control passages because they control the positioning of piston 320. By applying suction to one of the control passages and at the same time venting the remaining passages to atmosphere, the piston can be moved so that one of its Working surfaces is brought into cooperative engagement with the wall through which the suction control signal is being applied.

Each wall may be provided with one or more passages for producing a vacuum signal indicating the position of the piston. Thus, wall 330 is provided with a passage 332 for producing a suction output signal when the piston 320 is moved so that its working surface 334 is in cooperative engagement with wall 330. In like manner, passages 336, 338, and 340 are provided for producing a vacuum output signal when working surface 342, 344, or 346 is in cooperative engagement with wall 348, 350, or 352, respectively.

In addition to the inlet and outlet passages mentioned above, each wall may have a plurality of inlet and outlet passages for receiving and transmitting either vacuum or positive pressure signals indicating the state of the piston. These passages are designated 350 through 357. The manner in which they function will be explained subsequently.

The gating element 310 is also provided with vent passages for continuously connecting each corner of chamber 318 to the atmosphere regardless of the position of piston 320. Four of these vent passages 358, 360, 362, and 364 are shown in FIGURE 8.

The method of sensing the piston position as described with reference to FIGURES 3 and 4 is not suitable for sensing the position of piston 320 because it would permit the piston positioning signals to enter the position sensing outlet. To overcome this difliculty a different sensing method is employed.

As shown in FIGURE 9 wall surface 330 has a plurality of divider elements 366, 368, and 370 thereon. These divider elements are elongated ridges extending across the width of surface 330. Each ridge extends outwardly the same distance from surface 330 so that when piston surface 334 is drawn toward -wall 330 the ridges together with the piston and wall surfaces form two fluid chambers. The ridges function as fluid seals to prevent the leakage of fluid into or out of the chambers thus formed. FIGURE 8 shows the two chambers 372 and 374 formed when piston 320 is moved so that working surface 344 is in cooperative engagement with the fluid seals extending from wall 350.

Assume that piston 320' is in any stable state other than the one shown in FIGURE 8. If a vacuum signal is applied to control passage 326 and at the same time the control passages 322, 324, and 328 as well as the two control passages not shown are all vented to the atmosphere, the resulting force moves piston 320 toward wall 350 until working surface 344 contacts the seals. This forms two separate fluid passages or chambers 372 and 374.

The vacuum signal applied to passage 326 passes through chamber 374 and is available in outlet passage 338 as a vacuum signal indicating the state of the piston. Either a vacuum signal or a fluid pressure signal at greater than atmospheric pressure may be applied over passage 355 and this signal passes through chamber 372 to become available as an output signal in passage 354 indicating the position of the piston.

If the pressure signal applied to passage 355 is greater than atmospheric pressure then its magnitude must be limited so that it will not force the piston away from the fluid seals. On the other hand, if the pressure signal applied to passage 355 is a vacuum signal then the magnitude should be small enough to insure that it will not draw the piston against the fluid seals.

As long as the piston is held in the position shown, sensing signals applied to inlet passages in the other walls are not transmitted to outlet passages. For example, with piston 320 in the position shown, variations in the fluid pressure in passage 351 are not transmitted to passage 350. The reason is that the pressure signal is vented to the atmosphere through vents 358, 360, etc.

It will be understood that although piston 320 has been described as being cube-shaped, it is possible for each of the dimensions to be different. Furthermore, the piston may have any number of Working surfaces and corresponding stable states and is not limited to polyhedrons having six sides.

While the basic principles of the invention have been disclosed in connection with various specific embodiments, it will be understood that various modifications may be made in the devices shown without departing from the spirit of the invention. For example, the sensing method disclosed in FIGURES 8 and 9 may be employed to sense the position of pistons having two degrees of freedom. When this is done then the piston chamber need not have its corners filled as shown in FIGURE 2A. The corners are filled to provide positive and exact positioning of the piston so that the hole in the piston is properly aligned with the outlet passages. When the sensing system of FIGURES 8 and 9 is employed this alignment is not necessary so that corners of the chamber need not be fillcd. It is intended therefore to be limited only by the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The combination comprising: a piston element having N working surfaces disposed in N planes, at least two of said planes being non-parallel and N being a whole number greater than two; means defining a piston chamber having N walls each disposed substantially parallel to but spaced apart from a corresponding working surface of said piston when the piston is centered in said chamber; said piston having M stable states in each of which one of said working surfaces is in cooperative engagement with its corresponding chamber wall, M being any whole number greater than one but equal to or less than N; a plurality of fluid passages extending through the walls of said chamber, there being at least one passage extending through each of said walls; means for selectively creating a fluid pressure differential between one of said passages and the remainder of said passages to thereby produce fluid forces acting on the working surfaces of said piston so as to move said piston toward the one said passage and thereby into cooperative engagement with the corresponding chamber wall; and means for sensing the position of said piston within said chamber to indicate the state thereof.

2. The combination as claimed in claim 1 wherein said means for selectively creating a fluid pressure differential comprises a plurality of sources for selectively producing fluid pressure signals of a first or a second level, and fluid conducting means connecting each source to 8 said chamber through a corresponding passage in said walls.

3. The combination as claimed in claim 2 wherein said piston has M:NK stable states, K being the number of working surfaces to which fluid pressure signals of said second level are not selectively applied.

4. The combination as claimed in claim 1 wherein said piston has two sliding surfaces and said means defining said chamber includes two surfaces in sliding contact with the sliding Surfaces of said piston whereby said piston has two degrees of freedom and moves in a single plane between said stable states.

5. The combination as claimed in claim 4 wherein said sensing means comprises a first fluid passage terminating at a first port centrally located in one sliding surface of said chamber, a recess in one sliding surface of said piston, said recess being large enough to mate with said first port regardless of the position of said piston; a second passage extending from said recess through said piston to the other sliding surface of said piston; M output passages terminating at ports in the other sliding surface of said chamber, said output ports being positioned to individually mate with said second passage when said piston is in one of said stable states; and means for applying sensing signals to said first passage.

6. The combination as claimed in claim 1 wherein the length, depth, and width of said chamber are all greater than the corresponding dimensions of said piston whereby said piston has three degrees of freedom.

7. The combination as claimed in claim 1 and further including a plurality of fluid seals disposed on each of said chamber walls, one of said walls, its corresponding working surface, and the seals thereon, forming a plurality of signal chambers when said piston is in one of its stable states; said means for sensing the state of said piston comprising at least first and second fluid passages terminating at ports in one of said signal chambers, means for applying a fluid signal to said first passage, and output means connected to said second passage.

8. The combination comprising: a piston element having first and second triangularly shaped sliding surfaces and three working surfaces; means defining a piston chamher having first and second surfaces in sliding contact with the Sliding surfaces of said piston; said piston chamber having three walls corresponding to, disposed parallel to, and spaced from said working surface thus defining a chamber larger than said piston whereby said piston has two degrees of freedom of movement in a single plane; means for applying fluid signals to said chamber to selectively move one of said working surfaces against its corresponding piston wall; and means for sensing the position of said piston.

9. The combination as claimed in claim 8 wherein said sensing means comprises: an inlet terminating at an opening in one sliding surface of said chamber; a plurality of outlets terminating at ports in the other sliding surface of said chamber; and a fluid passage extending through said piston from said first to said second sliding surface.

10. The combination comprising: a piston element having first and second rectangularly shaped surfaces and four working surfaces; a piston chamber having first and second surfaces in sliding contact with the sliding surfaces of said piston; said piston chamber having four walls corresponding to, disposed parallel to, and spaced from said working surfaces thus defining a chamber larger than said piston whereby said piston has two degrees of freedom of movement in a single plane; means for applying fluid signals to said chamber to selectively move one of said working surfaces against its corresponding piston wall; said last-named means including a plurality of fluid passages extending through the Walls of said chamber, there being at least one passage extending through each of said walls, and means for selectively creating a fluid pressure differential between one of said passages and the remainder of said passages; and means for sensing the position of said piston.

11. The combination as claimed in claim 10 wherein said sensing means comprises: an inlet terminating at an opening in one sliding surface of said chamber; a plurality of outlets terminating at ports in the other sliding surface of said chamber; and a fluid passage extending through said piston from said first to said second sliding surface.

12. The combination as claimed in claim 9 wherein said means for applying fluid signals comprises: a plurality of passages each terminating at a port in one of said chamber walls; and means for selectively applying a fluid pressure signal of a first level to one of said plurality of passages while applying fluid pressure signals of a second level to the others of said plurality of passages.

References Cited UNITED STATES PATENTS 1,998,441 4/1935 Campbell 92 177 3,151,623 10/1964 Riordan 137 112 WILLIAM F. ODEA, Primary Examiner.

ALAN COHAN, Examiner.

H. COHN, Assistant Examiner. 

1. THE COMBINATION COMPRISING: A PISTON ELEMENT HAVIN N WORKING SURFACES DISPOSED IN N PLANES, AT LEAST TWO OF SAID PLANES BEING NON-PARALLEL AND N BEING A WHOLE NUMBER GREATER THAN TWO; MEANS DEFINING A PISTON CHAMBER HAVING N WALLS EACH DISPOSED SUBSTANTIALLY PARALLEL TO BUT SPACED APART FROM A CORRESPONDING WORKING SURFACE OF SAID PISTON WHEN THE PISTON IS CENTERED IN SAID CHAMBER; SAID PISTON HAVING M STABLE STATES IN EACH OF WHICH ONE OF SAID WORKING SURFACES IS IN COOPERATIVE ENGAGEMENT WITH ITS CORRESPONDING CHAMBER WALL, M BEING ANY WHOLE NUMBER GREATER THAN ONE BUT EQUAL TO OR LESS THAN N; A PLURALITY OF FLUID PASSAGES EXTENDING THROUGH THE WALLS OF SAID CHAMBER, THERE BEING AT LEAST ONE PASSAGE EXTENDING THROUGH EACH OF SAID WALLS; MEANS FOR SELECTIVELY CREATING A FLUID PRESSURE DIFFERENTIAL BETWEEN ONE OF SAID PASSAGES AND THE REMAINDER TO SAID PASSAGES TO THEREBY PRODUCE FLUID FORCES ACTING ON THE WORKING SURFACES OF SAID PISTON SO AS TO MOVE SAID PISTON TOWARD THE ONE SAID PASSAGE AND THEREBY INTO COOPERATIVE ENGAGEMENT WITH THE CORRESPONDING CHAMBER WALL; AND MEANS FOR SENSING THE POSITION OF SAID PISTON WITHIN SAID CHAMBER TO INDICATE THE STATE THEREOF. 